Low voltage differential signal (LVDS) interface systems capable of operating with high speed, low power consumption and/or low electromagnetic interference (EMI) characteristics are used in various applications, such as semiconductor devices, liquid crystal displays, communication systems, and the like.
FIG. 1 is a circuit diagram illustrating a conventional interface system 10 using low voltage differential signals.
Referring to FIG. 1, the conventional interface system 10 using low voltage differential signals includes an LVDS driver 20 and an LVDS receiver 30. The LVDS driver 20 generates an LVDS pair with a first signal Vin1 and a second signal Vin2, which is an inverted signal from the first signal Vin1. The LVDS receiver 30 includes a receiving unit 31 that reconstructs the first signal by detecting a voltage difference between received signals.
The LVDS driver 20 and the LVDS receiver 30 may respectively be formed as integrated chips, and a termination resistor RL may be attached as an external element between the LVDS driver 20 and the LVDS receiver 30.
The LVDS driver 20 includes a current source 21 for supplying a given amount, for example, about 3.5 mA, of current, and first, second, third and fourth transistors M10, M20, M30 and M40.
When a level of the first signal Vin1 is low and a level of the second signal Vin2 is high, the first transistor M10 and the fourth transistor M40 are turned on, and current flows in one direction through the termination resistor RL. On the other hand, when a level of the first signal Vin1 is high and a level of the second signal Vin2 is low, the second transistor M20 and the third transistor M30 are turned on, and the current flows in the opposite direction through the termination resistor RL.
Low voltage differential signals appear at each end of the termination resistor RL. A voltage developed across the termination resistor RL may be about 350 mV when the current supplied by the current source 21 is about 3.5 mA and the resistance of the termination resistor RL is about 100 ohms.
The low voltage differential signals produce a voltage across the termination resistor RL in front of the LVDS receiver 30. Therefore, the termination resistor RL may be used in constructing a conventional interface system 10 using low voltage differential signals.
Conventionally, termination resistors are added as external elements to the LVDS receiver. The external termination resistors may maintain the accuracy of the received signals within a range of about 1%. In contrast, termination resistors that are formed integral to an LVDS receiver (e.g. on the same integrated chip as the LVDS receiver 30) may typically maintain the accuracy of the received signals in a range of about 20%. Thus, it has generally been advantageous to use external termination resistors instead of integrated termination resistors.
External termination resistors have some drawbacks, however. For example, while an LVDS driver or receiver may be implemented as an integrated circuit inside a chip, the external termination resistors are not implemented inside the chip, which may impair signal accuracy due to impedance mismatches and/or bad hardwiring. An interface system with external termination resistors may cause distorted signals to be applied to the LVDS receiver, and/or may have poor jitter characteristics.
Furthermore, external termination resistors are typically installed on a printed circuit board (PCB) one by one, resulting in high manufacturing costs and/or complicating the manufacturing process. The more signal transfer channels the interface 10 has, the greater the problems become.
In addition, because the resistances of the external termination resistors are fixed, if the resistances need to be adjusted, the external termination resistors may be replaced with new external termination resistors one by one.
As discussed above, external termination resistors may have performance limitations, high costs, and/or complex installation processes, and the resistances of external termination resistors may not be easily adjusted.
Internal termination resistors integrated with an LVDS receiver have been used to address some of these drawbacks. However, internal termination resistors may suffer significant degradation in signal accuracy.